I think if your going to be there over 12 months you will have to get an international driving licence
No, you don't. As long as you aren't in Australia on a permanent visa, you can drive with your overseas drivers license indefinitely. (And WHVs are temporary visas, even if you take two in a row, so this applies.)
The rules are slightly different for each state, but this they all agree on. See for example Victoria: "If you are in Victoria on a temporary visa, you can drive on your overseas driver licence for as long as it is current "
(Also, international driving "licenses" (as far as I know about them being offered) are effectively a scam. An international driving permit isn't, but there's very few countries in the world actually requiring one of those.)
[ Edit: Edited on 08-Dec-2010, at 14:38 by Sander ]